Diode bars are high-power semiconductor lasers containing a one-dimensional array of broad-area emitters or, alternatively, subarrays containing 10-20 narrow stripes. Diode bars typically contain between 20 and 50 emitters, each being about 100 μm wide. A typical commercial device may comprise a laser resonator length on the order of 1-5 mm, with a width of about 10 mm. This size of device is capable of generating tens to hundreds of watts of output power without much difficulty. Power levels up to 250 W continuous output power are commercially available.
Electrically, the set of emitter regions are all connected in parallel, thus requiring a substantial drive current (particularly for configurations of 50 emitters or more), and may be on the order of tens (or even hundreds) of amps, with a rough rule-of-thumb being that one watt of output power is provided for every amp of input current. Important design parameters of diode bars are based upon the numbers of emitters, their width and spacing. With respect to beam quality and brightness, it is ideal to obtain the output power from a small number of closely-spaced emitters.
The need to operate at high powers results in a significant quantity of heat being generated. In order to maintain acceptable performance of the diode laser over a reasonable lifespan (say, 20 kh or more), the diode bar needs to be mounted onto a heatsink or provided in some type of arrangement with a low thermal resistance. Conventional heatsink materials (such as copper) exhibit a coefficient of thermal expansion (CTE) that is different from the CTE of the diode laser bar itself. The mis-match in CTE results in creating mechanical stress between the heatsink and diode laser bar during the high-temperature process of attaching the diode laser bar to the heatsink. At the elevated temperature required for the soldering process, the heatsink expands to a greater extent than the diode laser bar. As the combination returns to room temperature and both the heatsink and diode laser bar contract, the heatsink will contract more than the diode laser bar, creating a mechanical stress at their interface which is considered to be “frozen” into the configuration. While operational temperatures are somewhat elevated compared to room temperature, only a slight amount of stress relief may be expected.
The presence of the stress results in the formation of a curvature along the lateral extent of the diode laser bar (also referred to at times as a “smile”). This unwanted smile artifact is known have detrimental effects on the ability to focus beams from diode bars.
Thus, a need remains for a diode laser bar assembly that is able to accommodate the changes in stress associated CTE mis-match, while also maintaining a relatively low value of thermal resistivity.